Last Moon of Summer

Moon and Sun approach opposite crossroads in the sky.

The Moon will reach its Full position on Friday Sep. 16, at about 19 hours Universal Time. That is 3 PM by clocks in eastern North America, 2 PM in the Central and 12 noon in the Pacific zones, so we show the scene in the following evening twilight.

The Moon reaches its Full state a day before reaching longitude zero, the vernal equinox point or “First Point of Aries” where the ecliptic and celestial equator cross. But it is riding more than its own width south of the ecliptic, so it almost misses the shadow of Earth cast by the Sun – but not quite, as shown in this diagram from the “Eclipses” section of Astronomical Calendar 2016.

This is the third, and second slightest, of this year’s three penumbral lunar eclipses: Earth’s pale outermost shadow grazes the Moon’s northern edge. Since the moment of Full Moon happens earlier than the time pictured, the eclipse has already passed: it was seen – or, much more likely, not seen – over the Indian Ocean and surrounding lands.

Almost simultaneously with the Full-Moon-and-eclipse moment (only 5 hours earlier) the Sun is entering the constellation Virgo. The Moon, on the opposite side of the celestial sphere, is about to enter the opposite constellation, Pisces. Six days later will come the moment when the Sun reaches the actual crossroads of ecliptic and equator: the autumn equinox.

So on Friday evening, as you watch the Moon sloping up from the eastern horizon, you can feel the Sun sliding below the horizon directly behind you. They are, for a moment, the two pans of a balance, or the wingtips of a turning eagle. To your right, in the south, Mars and Saturn still hang in front of the Milky Way.